This invention is related to waste incinerators such as are used in numerous municipalities, and in industrial/commercial operations. More particularly it relates to an inclined hearth, and particularly to a multiple stepped hearth or stepped floor incinerator into which waste to be incinerated is loaded at one end, and incinerated waste residue including ashes is removed from the other. By "waste" I refer to municipal solid waste, and industrial/commercial solid waste, optionally mixed with sludge. Municipal solid waste includes garbage and refuse, scrap paper, glass scrap and metals of the common types, many of which may melt, like glass, at incineration temperatures generated in the incinerator. Industrial/commercial solid waste includes cardboard and other paper products, packing materials of all types, wood, plastic and glass scrap, and the like. Sludge is typically semi-solid waste such as is generated in paint plants, paper mills and the like, or in municipal water treatment facilities, and if sludge is to be incinerated it will generally be mixed with the foregoing solid wastes. All the foregoing wastes including sludge are referred to hereafter as "waste".
Incinerators of this general type are disclosed in U.S. Pat. Nos. 4,172,425, and 3,812,794 inter alia. They are "mass burning", that is, the heterogeneous waste is burned essentially as received from the typical municipal garbage truck, rubbish disposal truck, or industrial plant. The waste may be unprocessed, or processed before it is incinerated. Incinerating this hetrogeneous waste successfully has been the goal of numerous incinerator designs, some of which are referenced in the '425 patent. These designs include a variety of grate configurations designed to move, advance, shuffle, tumble or otherwise mix and agitate the burning waste so as to provide better incineration due to better distribution of primary air.
All modern incinerators have systems for the distribution of combustion air to the waste. Commonly this distribution is in the form of overfire combustion air (above waste bed) and underfire combustion air (below waste bed), although some designs preclude the use of overfire combustion air.
Underfire combustion air has typically been supplied through a "grate" system comprising cast iron or alloy metal shapes through which the air passes, similar to the grates utilized in a coal-fired boiler. This air cools the grates in addition to providing oxygen for combustion.
Other designs utilize refractory hearths in place of the grates with air distribution through holes or through metallic pipes with holes in them.
Both underfire combustion air designs are subject to plugging of the air passages and holes with molten materials such as glass, metal, plastics and mixtures of these with ash, metal, etc. This pluggage reduces the flow of air resulting in lower combustion rates and poor "burnout" of the combustibles. In addition, once the molten materials have cooled, a labor-intensive operation is required to clean out the air passages or holes. Flammable molten materials may flow through the passages and burn below the grates causing thermal damage.
This invention is primarily directed to stepped hearth, controlled combustion air incinerators of the general type disclosed in the '425 patent and the references cited therein. This invention is not especially well suited for adaptation in "grate" incinerators.
A stepped hearth incinerator conventionally comprises an elongated housing defining a combustion chamber the floor of which comprises stepped hearths on which combustion of the waste occurs. Means are provided for loading the waste through one end wall of the combustion chamber onto the loading hearth which is uppermost in the floor of stepped hearths in the combustion chamber. Further, ram means are provided for advancing solid waste over at least the first (uppermost) hearth to provide controlled advancement of the waste during the initial stage of its incineration, and to avoid the sudden and sporadic movement of large piles of waste, which movement would occur if no ram means were used.
By "ram means" I refer to any member which is longitudinally reciprocable against waste within a zone in the incinerator, and which member has the effect of advancing the waste through that zone. The constructional details of the ram means are not critical, are well known to those skilled in the art, and are disclosed in the '425 patent, inter alia, along with various other conventional structural details and factual statements with regard to the general considerations pertinent to the operation of the incinerator. Such common facts and analogous structural details are incorporated by reference thereto as if fully set forth herein, so as not to burden this specification unnecessarily.
In particular, air injection nozzles used in the '425 invention are carried by the ram; and these nozzles which inject air into the burning waste are susceptible to pluggage by molten debris and ash. The problems endemic to the operation of prior art systems provided the impetus to dissociate the air injection means and ram means; to eschew the use of perforated nozzles; to protect air feed-tubes from the high temperatures generated during combustion; to provide a positive clean-out for the air feed-tubes, to keep their ends in the burning waste open; and, to provide means for effectively cleaning passages in hearths and sidewalls.
The significance of my invention will be appreciated when it is realized that in general, incinerator modules burning municipal solid waste require more maintenance than those burning industrial refuse. In addition, more operational interruptions must be anticipated when burning municipal waste, because of the jams caused by large metal objects in the waste and the greater frequency of routine maintenance. Thus minimizing labor is of great importance. In particular, the module of the '425 type, because of its more extensive control system, required more maintenance than expected on the automatic control, hydraulic and residual removal system (see "Small Modular Incinerator Systems with Heat Recovery: A Technical, Environmental and Economical Evaluation" by Richard Frounfelker, U.S. Environmental Protection Agency 1979).